In the fabrication of semiconductors, a number of different types of unwanted films, particles, and residues are formed or are left on the edges of silicon wafers. The relevant edges include the top (or front), top bevel, apex (or sidewall), bottom bevel, and bottom (or back) edges of wafers (hereinafter “wafer edges”). These edges are further specified in the International Standards published by Semiconductor Equipment and Materials International (SEMI) of San Jose, Calif. and referred to as the SEMI M1-1109 and SEMI M73-0309 specification documents.
Many types of processes are used that result in such unwanted materials, including photoresist coating, plasma etching, dielectric and metal deposition, wet etching, grinding, and chemical mechanical polishing (CMP). These processes all result in various films, residues, and particles being deposited or left on the wafer edges.
The unwanted materials on wafer edges may be organic polymer films, such as photoresist edge beads, or residues, such as post-etch polymer films that remain on the edge of a wafer after a plasma etching operation. Other types of unwanted materials on wafer edges include metals, ‘black silicon’, silicon particles, and polishing residues.
During semiconductor manufacturing, wafers are transported from one tool to another in cassettes, and once loaded into a tool, are moved between process stations with robots handlers. As a consequence, films at the very edges of wafers become abraded, cracked, and broken. Cracked film fragments and particles contaminate vacuum chucks, robotic handlers, and cassettes, causing tool down time and die yield loss. Backside wafer contamination may distort or tilt the wafer during subsequent imaging, also causing yield loss.
Another source of wafer edge defects is the formation of a photoresist edge bead, caused when the wafers are spin coated prior to lithographic imaging. Photoresist spin coating is repeated many times in the fabrication of an integrated circuit chip, depending on the number of lithographic levels used to make the device. A thickened bead of photoresist is formed at the outermost top edge. Excess photoresist also migrates to the apex, beveled edges, and bottom edge of the wafer. The photoresist bead on the edge is easily cracked by robots handling the wafers, resulting in flakes of photoresist re-depositing on good die, causing yield loss. Any photoresist remaining on the wafer's bottom edge also sticks to vacuum chucks, causing leveling and distortion problems in lithography, and creating more cracked flakes of photoresist. Two methods are currently used for the removal of photoresist edge bead films. The first method is called wafer edge exposure and development (WEE), a two-step process described below in a prior art patent. The second conventional method for photoresist removal from wafer edges is solvent spraying or solvent cleaning.
Another example is residue left behind from plasma etching. These etch residues are thin, tough films such as halogenated polymers. They are deposited mainly on the wafer's bevel, apex, and bottom edges. They cannot be removed by conventional wet chemical means, and are currently removed with a grinder, in combination with a fluid to make a slurry. This is a messy and particle-generating process that leaves many defects behind. If these etch residues are not removed, they will crack, break up and leave particles on otherwise usable portions of the wafer, causing die loss.
Another example of an edge-cleaning problem is the removal of copper films from wafer edges, left over from copper metallization and etching steps. Remaining copper films on the wafer edges will cause electrical shorting and arcing during subsequent wafer processing steps. Copper films left on wafer edges must removed by a complex etching operation, for example wet etching followed by high purity water rinsing and drying steps.
Another edge cleaning problem is the treatment of ‘black silicon’ needle-like structures that are the result of a reactive ion etch (RIE) process. These structures are currently removed by a wet-etch process using hydrofluoric acid, followed by high purity water rinsing and drying steps.
Another example of an edge cleaning problem is the generation of silicon particles from edge thinning operations, commonly used in 3D interconnect devices. Grinding with abrasive wheels leaves large ridges of silicon that interfere with wafer-to-wafer bonding operations. Currently, the only method of removing these ridges is by chemical etching with hydrofluoric acid and water rinsing.
Another example of an edge-cleaning problem is defects resulting from chemical-mechanical polishing (CMP) residue. CMP slurries flow around the apex of wafers and onto the bottom edges. These residues and particles migrate onto vacuum chucks, robotic handlers, and other wafer processing equipment, again leading to die loss.
Yet another example of an edge-cleaning problem arises from the use of silicon-containing photoresist such as in tri-layer film stacks, consisting of an organic polymer layer, a carbon-based layer, and a silicon-containing layer. The silicon content can typically be up to 50% of the polymer content, and conventional removal methods, such as wafer edge exposure and development (WEE) or solvent cleaning, will not completely remove these silicon based films. Removal of silicon layers is currently performed in an etching operation with a reactive ion etcher system using fluorinated gas mixtures, followed by considerable water rinsing and drying steps.